This invention relates to an improved process for the preparation of polynuclear aromatic polyamines in which the reaction product is prepared by condensing aniline with formaldehyde in the presence of water and acidic catalysts and is worked up by extraction with a hydrophobic solvent. The acid catalyst that accumulates in the aqueous phase during the extraction process is reused.
It is already known that in the preparation of polynuclear aromatic polyamines by condensation of aniline with formaldehyde in the presence of water and acidic catalysts, the accumulating aqueous reaction mixture can be worked up by extraction with a hydrophobic solvent and that the acid catalyst that accumulates during extraction in the aqueous phase can be reused. See, for example, U.S. Pat. Nos. 4,093,658, 4,087,459, 4,061,678, 3,996,283, and 3,952,042 and DE-OS No. 2,343,658.
The main advantage of the processes according to the above references is the absence of the need to neutralize the catalyst. The catalyst need not be neutralized because it accumulates in the aqueous phase during workup of the acidic reaction mixture by extraction and may be returned to the beginning of the process and reused. In addition, certain variations of this known principle, such as those described in U.S. Pat. Nos. 4,093,658 or 4,087,459, provide for the specific preparation of polyamine mixtures with either an increased or reduced content of 2,4'-isomers. The products obtained by the processes disclosed in the above references correspond in suitability as starting products for the preparation of polyisocyanates to the conventional polyamines of the diphenylmethane series produced by neutralizing the acid catalyst. Thus, the property level of polyurethane foams produced from such polyisocyanate mixtures of the diphenylmethane series is substantially the same in both cases. However, a disadvantage of the processes according to the references cited above is that considerable quantities of hydrophobic solvent and aniline must be used solely for the extraction used for the workup of the end products. As a consequence, these processes involve a considerable amount of distillation in the workup of the organic phase and hence a considerable consumption of energy.
The object of the present invention is to provide a new, improved process for the preparation of polynuclear aromatic polyamines from aniline and formaldehyde in the presence of acidic catalysts. The improved process is intended to improve upon the advantages of the known processes and, in addition, to provide for the preparation of products of improved quality with less distillation and, hence, lower energy consumption. This object is achieved by the process according to the invention which is described in detail hereinafter.